Many modern mobile devices such as smartphones contain processors with speeds exceeding 600 MHz, and it is contemplated that future generation products will be even faster, and will have even more processing capabilities. In order to fully utilize the processing capabilities of such a device, certain applications need to send and receive large amounts of data over a wireless network. In order to maximize both the mobility of the device and the processing capabilities of the device, the wireless network should have both a large range and a high data transfer rate. One common solution used in the art is to have the device connect to a cellular phone network such as CDMA, GSM, GPRS, or EDGE, or to a wireless network such as a WiFi network. Both these solutions, however, have limitations. Cellular phone networks do not provide a high enough data transfer rate to run certain applications, and WiFi networks have limited ranges (typically less than 200 meters), significantly limiting the mobility of a device.
In light of the shortcomings of the aforementioned network architectures, a new wireless network protocol called Worldwide Interoperability for Microwave Access or “WiMAX” has emerged. WiMAX has a range comparable to cellular phone networks and data transfer rates comparable to WiFi networks. The base stations in a WiMAX network can frequently transmit data over a greater than 10-mile radius. For all transmissions, the IEEE 802.16 standard governing the WiMAX protocol dictates that the mobile station employ relative transmit power levels selected according to a normalized carrier-to-noise (C/N) modulation table, shown in Table 1 below. Referring to Table 1, a normalized C/N value is specified and used relatively for each combination of modulation mode and forward error correction (FEC) rate. That is, the absolute transmit power levels are selected according to the differences between the normalized C/N values shown in Table 1.
TABLE 1Modulation/FEC rateNormalized C/N (dB)ACK region−3Fast-feedback0Ranging region3QPSK 1/30.5QPSK 1/26QPSK 2/37.5QPSK 3/49Sounding916-QAM 1/21216-QAM 2/314.516-QAM 3/41516-QAM 5/617.564-QAM 1/21864-QAM 2/32064-QAM 3/42164-QAM 5/623
FIG. 1 shows a prior art process 100 for a mobile device to establish an initial connection with a WiMAX base station. The WiMAX base station transmits a broadcast message containing information identifying the base station and other network information. After scanning downlink channels and detecting the broadcast message (step 102), the mobile device transmits a known sequence called a ranging sequence to the WiMAX base station (step 104). The mobile device generally transmits the ranging sequence at some power level and stores that power level for future reference.
The base station responds with an anonymous ranging response message. After receiving the anonymous ranging response message (step 106), the mobile device can transmit a ranging request message (step 108). The mobile device generally transmits the ranging request message at a power level selected according to the “QPSK ½” entry in Table 1, which specifies a C/N value of 6 dB, which is 3 dB higher than the value of the “Ranging region” entry. In response, the base station transmits a ranging response message with basic channel identifier (CID). If the mobile device receives the ranging response message with basic CID (step 110), then the mobile device can begin exchanging regular traffic according to the modulation modes and C/N values of Table 1 (step 112). However, if the mobile device does not receive the ranging response message with basic CID within a specified interval (step 110), the mobile device transmits the ranging sequence again (step 104). After a predetermined number of attempts with no response (step 114) the mobile station returns to scanning downlink channels (step 102). One disadvantage of these approaches is long network entry times due to repeated unsuccessful ranging attempts.
An alternative approach is to allow the mobile device to increase transmission power by more than 3 dB (which is the difference between the “QPSK ½” and “Ranging region” entries in Table 1) when transmitting the ranging request message. This approach is specified in the IEEE 802.16 standard governing the WiMAX protocol. In addition, this increased power level is not used for subsequent messages such as regular traffic. One disadvantage of this approach is that a mobile device may be able to establish an initial connection with a base station, but with subsequent messages at the insufficient transmit power level, leading to a frustrating user experience.